As filed with the Securities and Exchange Commission on October 12, 2012

1933 Act Registration No. 333-162441

1940 Act Registration No. 811-22338

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM N-1A

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

x

Pre-Effective Amendment No.

¨

Post-Effective Amendment No. 50

x

and

REGISTRATION STATEMENT

UNDER

THE INVESTMENT COMPANY ACT OF 1940

x

Amendment No. 51

x

LEGG MASON GLOBAL ASSET MANAGEMENT TRUST

(Exact Name of Registrant as Specified in Charter)

100
International Drive

Baltimore, Maryland 21202

(Address of principal executive offices)

Registrants telephone number, including area code: (410) 539-0000

Name and address of agent for service:

Copy to:

RICHARD M. WACHTERMAN, ESQ.

ARTHUR C. DELIBERT, ESQ.

Legg Mason & Co., LLC

K&L Gates LLP

100 International Drive

1601 K Street, N.W.

Baltimore, Maryland 21202

Washington, D.C. 20006-1600

(Name and address of agent for service)

Approximate Date of Proposed Public Offering: Continuous

It is proposed that this filing will become effective:

x

immediately upon filing pursuant to Rule 485(b)

¨

on , pursuant to Rule 485(b)

¨

60 days after filing pursuant to Rule 485 (a)(1)

¨

on , pursuant to Rule 485 (a)(1)

¨

75 days after filing pursuant to Rule 485(a)(2)

¨

on, pursuant to Rule 485(a)(2)

If appropriate,
check the following box:

¨

This post-effective amendment designates a new effective date for a previously filed post-effective amendment.

This amendment is being filed solely to submit exhibits containing risk/return summary information in interactive data format that is identical to the
risk/return information contained in the Registrants prospectus for Legg Mason BW Global High Yield Fund which was filed with the Securities and Exchange Commission in Post-Effective Amendment No. 48 to the Registrants registration
statement on September 26, 2012.

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, as amended (the Securities Act), and the Investment Company
Act of 1940, as amended, the Registrant, LEGG MASON GLOBAL ASSET MANAGEMENT TRUST, hereby certifies that it meets all the requirements for effectiveness of this Post-Effective Amendment No. 50 to its Registration Statement under Rule 485(b)
under the Securities Act and has duly caused this Post-Effective Amendment No. 50 to its Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Baltimore, State of Maryland on this
12th day of October, 2012.

Pursuant to the requirements of the Securities Act, this Post-Effective Amendment to the Registration Statement has been signed below by the following persons in the capacities indicated below on
October 12, 2012.

Signature

Title

/s/ Mark R. Fetting*

Chairman and Trustee

Mark R. Fetting

/s/ R. Jay Gerken

President (Principal Executive Officer) and Trustee

R. Jay Gerken

/s/ Ruby P. Hearn*

Trustee

Ruby P. Hearn

/s/ Arnold L. Lehman *

Trustee

Arnold L. Lehman

/s/ Robin J.W. Masters *

Trustee

Robin J.W. Masters

/s/ Jill E. McGovern*

Trustee

Jill E. McGovern

/s/ Arthur S. Mehlman *

Trustee

Arthur S. Mehlman

/s/ G. Peter OBrien *

Trustee

G. Peter OBrien

/s/ S. Ford Rowan*

Trustee

S. Ford Rowan

/s/ Robert M. Tarola *

Trustee

Robert M. Tarola

/s/ Richard F. Sennett

Principal Financial and Accounting Officer

Richard F. Sennett

* By:

/s/ R. Jay Gerken

R. Jay Gerken

Attorney in Fact, pursuant to Power of Attorney filed herewith.

POWER OF ATTORNEY

I, the undersigned Director/Trustee of one or more of the following investment companies (as set forth in the companies Registration Statements on Form N-1A):

LEGG MASON INCOME TRUST, INC.

LEGG MASON CHARLES STREET TRUST, INC.

LEGG MASON GLOBAL TRUST, INC.

LEGG MASON GLOBAL ASSET MANAGEMENT TRUST

LEGG MASON TAX-FREE INCOME FUND

LEGG MASON INVESTORS TRUST, INC.

LEGG MASON CAPITAL MANAGEMENT GROWTH TRUST, INC.

LEGG MASON LIGHT STREET TRUST, INC.

LEGG MASON CAPITAL MANAGEMENT VALUE TRUST, INC.

LEGG MASON INVESTMENT TRUST, INC.

LEGG MASON INVESTMENT TRUST

LEGG MASON CAPITAL MANAGEMENT SPECIAL INVESTMENT TRUST, INC.

plus any other investment company for which Legg Mason Partners Fund Advisor, LLC or an affiliate thereof acts as
investment adviser or manager and for which the undersigned individual serves as Director/Trustee hereby severally constitute and appoint each of R. JAY GERKEN, RICHARD SENNETT, ERIN K. MORRIS, CHRISTOPHER BERARDUCCI, ROBERT I. FRENKEL, RICHARD M.
WACHTERMAN, THOMAS C. MANDIA, MARC A. DE OLIVERIA, MICHAEL KOCUR, ARTHUR C. DELIBERT and NDENISARYA M. BREGASI my true and lawful attorney-in-fact, with full power of substitution, and each with full power to sign for me and in my name in the
appropriate capacity and only for those companies described above for which I serve as Director/Trustee, any Registration Statements on Form N-1A, all Pre-Effective Amendments to any Registration Statements of the Funds, any and all Post-Effective
Amendments to said Registration Statements, and any and all supplements or other instruments in connection therewith, to file the same with the Securities and Exchange Commission and the securities regulators of appropriate states and territories,
and generally to do all such things in my name and behalf in connection therewith as said attorney-in-fact deems necessary or appropriate to comply with the provisions of the Securities Act of 1933 and the Investment Company Act of 1940, all related
requirements of the Securities and Exchange Commission and all requirements of appropriate states and territories. I hereby ratify and confirm all that said attorney-in-fact or their substitutes may do or cause to be done by virtue hereof.

Any subsequently executed power of attorney that grants powers unrelated to the powers granted herein will not revoke nor supersede this
power of attorney unless such subsequent power of attorney expressly states otherwise.